1. Field of the Invention
This invention relates to a connecting method for a structure for connecting electrode terminals of a liquid crystal display element and lead terminals of a flexible circuit board to each other by a pressuring and heating step.
2. Description of the Related Art
A flexible circuit board is normally connected to terminals of a liquid crystal display element by pressurizing and heating them using a heater chip or the like with a conducting bonding material such as an anisotropic conducting sheet interposed between them.
FIG. 2 shows a liquid crystal display element to which a plurality of flexible circuit boards are connected; FIG. 3 illustrates a positioning step upon connection of a flexible circuit board to the liquid crystal display element; and FIG. 4 illustrates a pressurizing and heating step for connecting the flexible circuit board to the liquid crystal display element.
Referring to FIGS. 2 to 4, the liquid crystal display unit generally denoted at 1 includes an upper glass substrate 2, a lower glass substrate 3, liquid crystal (not shown) enclosed between the two glass substrates 2 and 3, a polarizing plate 4 and so forth. Transparent electrodes (not shown) made of an ITO film or the like are formed in accordance with display patterns on opposing faces of the glass substrates 2 and 3. An end edge of the lower glass substrate 3 is formed as a terminal section 1a on which a large number of electrode terminals 5 for the external connection are formed in a juxtaposed relationship such that they extend individually from the transparent electrodes.
A known anisotropic conducting sheet 6 made of a thermosetting resin in which conducting powder is contained is used in order to establish electric and mechanical connection of two elements on the opposite faces thereof by pressurizing and heating. It is to be noted that such anisotropic conducting sheet is omitted in FIGS. 2 and 3 in order to avoid complexity in illustration.
A flexible circuit board is generally denoted at 7 and is formed by etching copper foil on a base film 8 made of a polyimide film or the like to form a conductor pattern and then coating the conductor pattern with a cover film 9. The conductor pattern is exposed at an end edge of the base film 8 as output lead terminals 10. An electronic part 11 such as an LSI is carried on the flexible circuit board 7.
In order to connect the flexible circuit board 7 to the liquid crystal display element 1, first the anisotropic conducting sheet 6 is placed onto the terminal section 1a of the liquid crystal display element 1, and then the mutually corresponding electrode terminals 5 and lead terminals 10 of the liquid crystal display element 1 and flexible circuit board 7 are positioned relative to each other as seen in FIG. 3. In this instance, a technique is normally employed in which the mutually corresponding electrode terminals 5 and lead terminals 10 are all registered with each other at the positioning stage by setting the terminal pitch p of the lead terminals 10 of the flexible circuit board 7 and the terminal pitch P of the electrode terminals 7 of the liquid crystal display element 1 equal to each other in advance. After the mutually corresponding electrode terminals 5 and lead electrodes 10 are positioned with a high degree of accuracy relative to each other, they are temporarily fixed to each other, and then they are normally fixed to each other by pressurizing and heating the flexible circuit board 7 on the terminal section 1a using a heater chip 12 as seen in FIG. 4 to melt and soften the anisotropic conducting sheet 6 to electrically and mechanically connect the mutually corresponding electrode terminals 5 and lead terminals 10 to each other.
However, in the series of steps described above, even if the electrode terminals 5 and the lead terminals 10 are positioned with a high degree of accuracy relative to each other upon temporary fixation, since the base film 8 of the flexible circuit board 7 is normally elongated a little upon normal fixation (at the pressurizing and heating step), the terminal pitch p of the lead terminals 10 will be a little greater than the terminal pitch P of the electrode terminals 5 after such normal fixation. Consequently, when the number of such terminals is great with a fine pitch, the reliability of the connection may possibly be deteriorated by displacement between the electrode terminals 5 and the lead terminals 10.